Laser barometer

ABSTRACT

A pressure measuring instrument that utilizes the change of the refractive index of a gas as a function of pressure and the coherent nature of a laser light to determine the barometric pressure within an environment. As the gas pressure in a closed environment varies, the index of refraction of the gas changes. The amount of change is a function of the gas pressure. By illuminating the gas with a laser light source, causing the wavelength of the light to change, pressure can be quantified by measuring the shift in fringes (alternating light and dark bands produced when coherent light is mixed) in an interferometer.

The United States Government has rights in this invention pursuant tocontract No. DE-AC04-90DP62349 between EG&G Rocky Flats, Inc. and theUnited States Department of Energy.

FIELD OF THE INVENTION

This invention relates to a pressure measuring instrument, and moreparticularly to a pressure measuring instrument which utilizes coherentlight from a laser to measure the pressure within an enclosedenvironment, accomplished by the change of the refractive index of a gasin an environment as a function of applied pressure.

BACKGROUND OF THE INVENTION

Pressure instruments that serve as a background for the invention can begrouped into three categories: general purpose pressure instruments;manometers; and piston gauges. General purpose pressure instruments aredevices such as Bourdon tube pressure gauges, capacitance diaphragmgauges and pressure transducers which are used to monitor/measureprocess parameters or as secondary standards used in the calibration ofprocess equipment. Bourdon tube gauges use an elastic tube which flexesas a function of the applied pressure. Capacitance diaphragm gauges usea moveable membrane which varies the capacitance of the sensing elementas a function of the applied pressure. Pressure transducers use a straingauge which changes value as a function of the applied pressure. Theseinstruments either use elastic elements or moving parts.

Manometers are liquid filled devices which measure pressure as afunction of the change in height of the column(s) of the liquid. Thesedevices can use water, alcohol, benzine, mercury or other fluids as themeasurement medium. The difference in the column heights is monitoredwith a scale or ruler calibrated in the pressure units of interest. Inthe most accurate namometers, lasers have been used to measure thecolumn heights. In this fluid based measurement system, it is necessaryto change fluids at specified intervals. Because alcohol, benzine andmercury are all RCRA (Resource Conservation Recovery Act) regulatedhazardous materials this procedure produces hazardous wastes.

Piston gauges are pressure measurement systems that measure pressurebased on the cross sectional area of a piston and an applied mass. Theseinstruments are typically used as primary pressure standards due to thefact that the measurement is based upon the physical quantities of massand area. During use, a mass consisting of one or more certified weightsis placed upon a weight table which is attached to the piston. Theapplied pressure is then increased or decreased through the use ofvalves and volume adjusters in order to place the piston and mass on afloat, the term float referring to a point when the piston is free tomove and encounters no friction forces from the upper or lower physicalrestraints used to hold the piston within its cylinder. Piston gauges ofthis caliber are manufactured to extremely tight tolerances. As with theother gauges, piston gauges contain moving parts.

Thus it is an object of the invention to provide a pressure measuringmeans free from moving parts or elastic elements.

It is another object of this invention to provide a pressure measuringmeans that reduces, or ultimately eliminates, the use of hazardous wastematerials.

It is yet another object of the invention to provide a pressuremeasuring means that does not require the use of weights to measurepressure.

These objects may be achieved by providing a pressure monitoringapparatus that utilizes changes in laser light energy as a result ofmodulations or changes in atmospheric pressure to accurately measurepressure within a system or environment.

SUMMARY OF INVENTION

To achieve the foregoing and other objects, and in accordance with thepurposes of the present invention as described herein, a novel apparatusfor measuring pressure within an environment is now presented. Thepressure measuring instrument described herein utilizes the change ofthe refractive index of a gas as a function of pressure and the coherentnature of a laser light to determine the pressure within a closedenvironment. For example, as the gas pressure in a closed tube varies,the index of refraction of the gas changes. The amount of the change inthe index of refraction is a function of the gas pressure. Byilluminating the gas with a laser light source, a change in pressurewithin the closed environment which causes the wavelength of the lightto change can be quantified by measuring the shift in fringes(alternating light and dark bands produced when coherent light is mixed)in an interferometer. This technique produces a primary measurementstandard because the resulting measurement is based upon the wavelengthof the laser light. The invention is useful in laboratories as a primarypressure standard to replace means such as existing mercury manometersto achieve accurate pressure measurements.

Still other objects of the present invention will become readilyapparent to those skilled in this art from the following detaileddescription wherein the preferred embodiment of the invention isdescribed. The invention will be set forth in part in the descriptionthat follows and in part will become apparent to those so skilled in theart upon examination of the following description or may be learned bypractice of the invention. Accordingly, the drawing and description willbe regarded as illustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing incorporated in and forming part of thespecification illustrates the present invention, and together with thedescription serve to explain the principles of the invention.

FIG. 1 illustrates a plan view of the laser barometer in a systemconfiguration.

DETAILED DESCRIPTION OF THE INVENTION

The purpose of this invention is to provide a pressure measurementapparatus that utilizes the change in the index of refraction of a gasas function of gas pressure and the coherent nature of the laser light.

Referring to FIG. 1, the laser barometer of the present invention isillustrated in a system configuration. In the preferred embodiment ofthe invention the basic components for the laser barometer include acomputer 2, laser electronics 4, a laser 6, a photodetector 8, ameasurement tube 10, an interferometer 12, reflectors 13-14, a vacuumpump 3, a gate valve 5, gas inlet port 7, and an optical window 9. Whenthe system as illustrated is in operation, the vacuum pump 3 is used toevacuate the measurement tube 10 in order to establish a referencepressure in the range of 10 mTorr. The laser 6 is turned on and the beampasses through the interferometer 12 where the beam is split into twoseparate components.

The first component is part of the laser beam that is directed at aright angle to the initial direction of the laser beam. The secondcomponent is the part of the laser beam that continues past theinterferometer 12. After the first component is split at theinterferometer 12 it is reflected off a side reflector 14 and backthrough the interferometer 12 into a photodetector 8.

The second beam component travels through the interferometer 12 towardsthe measurement tube 10 where the beam enters an optical window 9 andtravels through the measurement tube 10 to a reflector 13 at the back ofthe measurement tube 10. The beam is reflected by the reflector 14 atthe rear of the measurement tube 10 through the measurement tube 10,through the optical window 9, and back into the interferometer 12 whereit is directed in parallel to and mixed with the first beam component.

The mixing of the two beam components which are now out of phase withrespect to each other causes the generation of fringe lines within theinterferometer 12 and consequently by the photodetector 8. At this timethe zero pressure reference is established for the laser barometer 1.

After the zero reference is established, the gate valve 5 is closed inorder to isolate the vacuum pump 3 from the measurement tube 10. At thistime gas can be admitted through the gas inlet port 7. The admission ofgas into the measurement tube 10 causes the index of refraction (μ)within the measurement tube 10 to change. This change in μ isproportional to the gas pressure within the measurement tube 10. Thechange in μ causes the wavelength of the laser light to change which inturn causes the fringes formed in the interferometer 12 to shift. Thisshift in fringes is detected by the photodetector 8. The photodetector 8then transmits electronic pulses to the laser electronics 4 which areproportional to the number of fringes that shift past the sensingelement of the photodetector 8. The laser electronics 4 then transmitsthe information to the system computer 2 which performs the mathematicalconversions necessary to change the laser electronics information intopressure units.

Pressure is calculated by the computer using the following mathematicalmodel:$P = {\frac{N*\lambda}{m*l*\left( {1 + {\alpha*\left( {t - 20} \right)}} \right)}*\frac{\left( {1 + {a*t}} \right)*760}{\mu - 1}}$

Where:

P=the applied pressure (Torr)

N=the number of fringes detected by the photo detector

λ=the vacuum wavelength of the LASER

m=(the number of paths the light beam travels within the tube)

l=the length of the tube (meters)

a=the gas expansion coefficient

t=the temperature of the system (°C.)

α=the linear coefficient of expansion for the tube

μ=the index of refraction for the calibration gas at 0° C. and 760 mmHg

The addition of a second tube, a beam splitter and supportopto-electronics would allow the device to be used as a differentialpressure instrument. Also, by lengthening the tube or increasing thenumber of internal reflections within the tube, the sensitivity of thesystem can be increased.

The embodiments specifically disclosed herein were chosen and describedin order to best illustrate the principles of the invention and itspractical application to thereby enable one of ordinary skill in the artto best utilize the invention in various embodiments and with variousmodifications as are suited for the particular use contemplated. Otherembodiments of the invention will be apparent to those skilled in theart from a consideration of this specification and in practice of theinvention disclosed herein. It is intended that the specification andthe examples be considered as exemplary only, with the true scope andspirit of the invention being indicated in the following claims.

What is claimed is:
 1. A pressure measuring instrument comprising: a) ameans for generating a laser beam; b) a photodetector for receiving alaser beam; c) a container capable of containing diverse atmosphericenvironments, said container comprising an optical window on a first endof said container, and a reflecting means located at a second end ofsaid container, d) an interferometer for receiving said laser beam andsplitting said laser beam into first and second components, directingsaid first component to said photodetector wherein said first componentserves as a reference signal, and directing said second component tosaid container wherein said second component serves as a measuringsignal, and wherein said second component enters said container throughsaid optical window and is reflected by said reflecting means backthrough said container and said optical window to said interferometerwhere said second component is mixed with said first component and wheresaid mixing of first and second components which are out-of-phase withrespect to each other generates a series of fringes and where a shift insaid series of fringes generated within said interferometer is detectedby said photodetector; and where said photodetector outputs anelectronic pulse; and e) a computer means coupled to said photodetectorfor determining the calibration of a pressure based on said first andsecond components prior to introducing gas into said container, and fordetermining said pressure within said container once gas is introducedinto said container by converting said output electronic pulses intopressure units.
 2. The instrument of claim 1, further comprising; f) avacuum pump for evacuating said container prior to calibration or theintroduction of gas into said container; and g) an inlet means forintroducing said gas into said container after said calibration and saidevacuation.
 3. The invention of claim 2 wherein said inlet means forintroducing gas into said container is a valve means.
 4. A pressuremeasuring instrument comprising: a) a means for generating a laser beam;b) a photodetector for receiving a laser beam; c) a tube capable ofcontaining diverse atmospheric environments, said tube comprising anoptical window on a first end of said tube, and a reflecting meanslocated at a second end of said tube; d) an interferometer for receivingsaid laser beam and splitting said laser beam into first and secondcomponents, directing said first component to said photodetector whereinsaid first component serves as a reference signal, and directing saidsecond component to said tube wherein said second component serves as ameasuring signal, and wherein said second component enters said tubethrough said optical window and is reflected by said reflecting meansback through said tube and said optical window to said interferometerwhere said second component is mixed with said first component and wheresaid mixing of first and second components which are out-of-phase withrespect to each other generates a series of fringes and where a shift insaid series of fringes generated within said interferometer is detectedby said photodetector; and where said photodetector outputs anelectronic pulse; and e) a computer means coupled to said photodetectorfor determining the calibration of a pressure based on said first andsecond components prior to introducing gas into said tube, and fordetermining said pressure within said tube once gas is introduced intosaid tube by converting said output electronic pulses into pressureunits.
 5. The instrument of claim 4, further comprising: a vacuum pumpfor evacuating said tube prior to calibration or the introduction of gasinto said tube; and an inlet means for introducing said gas to bemeasured into said tube.
 6. The invention of claim 5 wherein said inletmeans for introducing gas into said tube is a valve means.